This invention relates to a process for hydrolyzing an acrylamide polymer, that is, a process for producing a partially hydrolyzed acrylamide polymer.
Acrylamide polymers, namely, homopolymers and copolymers of acrylamide have been widely used as flocculants or thickeners for paper making and more recently for secondary recovery of petroleum. Although the performance required of the polymer may differ depending on the respective purposes of uses, the common desirable features for these uses are most probably high molecular weight and good solubility of these polymers.
One group of such acrylamide polymers are copolymers of acrylamide with acrylic acid (or its salt) (comonomers are not necessarily limited to acrylic acid alone). One method for obtaining such a copolymer comprises hydrolyzing with an alkali agent a part of the amide groups by contacting an acrylamide polymer in the state of an aquagel obtained from the polymerization step. For example, the following proposals have been made: (1) a gel-like polymer is kneaded with an alkali agent by means of a rotary screw type extruder, and the kneaded mixture is dried according to necessity. (reference: Japanese Patent Publication No. 17668/1974); (2) granules of a gel-like polymer are contacted with an aqueous alkali, further subjected to regranulation by means of a granulating extruder and then dried. (reference: Japanese Laid-open Patent Publication No. 167705/1981); and (3) hydrous polymer particles are contacted with an alkali agent under agitation in a device equipped with a stirring mechanism with a ribbon type blade or a gate type blade whose surface to be in contact with the polymer particles is made of a synthetic resin, which step is followed subsequently by drying under heating (reference: Japanese Patent Publication No. 15793/1979).
All of these methods provide methods for mixing a gel-like polymer with an alkali agent and have successfully achieved their respective objects. To the best of our knowledge, however, these methods involve problems. That is, the methods (1) and (2) may be considered to be superior to the method (3) in the aspect of achieving uniform hydrolysis, but a lowering of the molecular weight of the polymer is unavoidable because of a great shearing force acting on the polymer in the presence of an alkali agent. On the other hand, according to the method of (3), while deterioration of the polymer is suppressed to some extent, it is not yet satisfactory with respect to carrying out hydrolysis uniformly. When uniform hydrolysis is not accomplished, various problems arise depending on the uses of the polymers obtained, even though the polymer may have a high molecular weight and good solubility. For example, when applied as a thickener for paper making, the pulp may undergo partial agglomeration to cause poor formation of the paper, sometimes resulting in failure of paper making in extreme cases. Also, when applied as an agent for secondary recovery of petroleum, white turbidity may be generated in water containing polyvalent metal ions to lower the function of the agent.
Still another disadvantage of the method for hydrolysis of the aquagel of an acrylamide polymer as described above is that, since a considerable part of the hydrolysis reaction is accomplished in the drying step, the by-produced ammonia is occluded in dried polymer particles, which ammonia is in turn released upon dissolution of the dried particles, whereby the working environment is impaired as a consequence of the generation of the odor of ammonia. In view of such a drawback, various proposals have also been made to use various additives (reference: Japanese Laid-open Patent Publication Nos. 102390-102392/1979).